This invention relates to image display systems in which image elements are projected onto a display surface such as a projection screen, the display surface of a cathode ray tube or, with appropriate precautions, directly onto the retina of the human eye.
Most direct-view video projector display systems are based on cathode ray tube (CRT) technology in which a beam of electrons is directed to impinge on a phosphorescent screen. The resolution and brightness of CRT technology-based displays are inherently limited by the characteristics of phosphors and electron beam control electronics.
Some projection systems have been built using light spot scanning techniques, as distinct from electron beam deflection techniques as used in CRTs. Scanning projection systems typically use two rotating mirrors, one for high speed horizontal scan and one for slower vertical scan. Such a system using two rotating multi-faceted mirrors is described, for example, in U.S. Pat. No. 2,163,548, issued in 1936. A modern variant is described in U.S. Pat. No. 4,097,115. Therein two mirrors are employed in the scanning system. A still further scanning system using rotating multi-faceted mirrors is described in U.S. Pat. No. 4,613,201. The spot scanning systems require expensive and precise high speed scanning mirrors. Typically such systems require mirrors with up to 36 precision faces with rotation speeds of 30,000 revolutions per minute in order to provide a system capable of realtime video projection. Such high-speed rotating mirrors typically require air-bearings or other expensive components. Accordingly, it is desirable to provide a large screen scanning system that eliminates the need for any rotating high speed multi-faceted mirrors or other elements which are particularly sensitive to failure or wear.
Laser illumination has been used in light scanning systems to illuminate a single pixel of image during a high-speed two-dimensional scan of a large projection screen system. A great deal of effort has been expended on achieving the objective using lasers because of their intense light and characteristic coherence. However, the use of lasers in wide spread commercial applications has been difficult to realize and practice for a number of reasons. Some of the difficulties have been discussed in a paper by C. E. Baker of Texas Instruments Inc., IEEE Spectrum, December 1968. One particular problem is the generally low efficiency of lasers, which results in unacceptably low picture brightness at large screen sizes for a given power consumption. An increase in laser output has involved an unacceptable increase in cost and complexity for all but the most cost-insensitive applications. Another problem involves the production of suitable red, blue and green light components for color displays. Consequently, systems employing multiple lasers have been required which results in increase in complexity and cost. Finally, a high-speed deflection technique is still required. Solutions offered have been a high-speed rotating multi-faceted scanning mirror, or alternatively, a Bragg cell.
White light sources have been found to be more suitable for color projection than lasers, which require a plurality of lasers to generate the desired colors. However, available light sources have the disadvantage of being less collimated and less coherent than a typical monochromatic laser. Thus, white light sources might be considered less than suitable for spot scanning. A white light source requires relatively large components, including concentration lens, modulator and the like in order to achieve a brightness comparable to that of a laser. Large optical components are unwieldy and expensive. Moreover, large modulators are characterized by speed limitations, making them unsuitable for highspeed scanning systems. Finely-focused bright white light sources are difficult to achieve and hence, resolution is limited. Accordingly, it is desirable to provide a system which neither requires a laser nor the large and expensive optical components which have in the past generally been associated with a white light scanning system. There are several alternatives to scanning-type systems. Among the alternatives are so called "spatial light modulators" (SLMs). One non-scanning color projection technology, referred to as "light valve technology", uses an oil film deformed by electron bombardment. This technology has been incorporated into two known commercially available systems, the Eidophor theatre projection system and the General Electric color television projector, as described in "Color Television Light Valve Projection Systems," IEEE International Convention, Session 26/1, 1-8 (1973) and "The Fischer Large-Screen Projection System (Eidophor)," by E. Baumann, 20 J. SMPTE 351 (1953). In the Eidophor and G.E. system, a continuous oil film is scanned in raster fashion with an electron beam that is modulated so as to create a spatially-periodic distribution of deposited charge within each resolvable pixel on the flexible film. This charge distribution results in the creation of a phase grating within each pixel by virtue of the electrostatic attraction between the oil film surface and the supporting substrate, which is maintained at constant potential. This attractive force causes the surface of the film to deform by an amount proportional to the quantity of deposited charge. The modulated light valve (incorporating the oil) is illuminated by spatially coherent light from an arc-lamp. Light which is incident to modulated pixels on the oil film is diffracted by the local phase gratings into a discrete set of regularly spaced orders which are made to fall on a Schlieren stop consisting of a periodic array of alternating clear and opaque bars by part of the optical system. The phase perturbations introduced by the modulated electron beam are thus converted into bright spots of light at the screen by the Schlieren projector. A number of non-oil film SLMs have been developed. Among such SLMs are those including deflectable elements, rotation of plane of polarization, and light scattering. Such SLMs employ various effects such as deformation of reflective layers of metal, elastomer, or elastomer-photoconductor, as well as polarization and scattering of ferroelectrics, PLZT ceramics, or liquid crystals. Prior systems have been disclosed by R. Sprague et al., "Linear Total Internal Reflection Spatial Light Modulator for Laser Printing," 299 Proc. SPIE 68 (1981) and W. Turner and R. Sprague, "Integrated Total Internal Reflection (TIR) Spatial Light Modulator for Laser Printing," 299 Proc. SPIE 76 (1991). Still further, SLMs are disclosed in U.S. Pat. No. 4,710,732. However, none of these technologies has resulted in high resolution, high brightness, video screen projection systems suitable for the consumer marketplace. An experimental video projector has been described by Murano et al. entitled "A Video Projector Using a PLZT Light Shutter Array," Japanese Journal of Applied Physics, 24 (1985) Supplement 24-2, pp. 139-143. Therein a line PLZT light shutter array was used to modulate light from a Xenon light source. The resulting modulated beam was scanned vertically by a movable mirror and then projected on the screen. However, this system experienced significant problems relating to brightness, uniformity of image, and mirror response time.
What is needed is a system for displaying an image from a sequence of intensity values representing pixels wherein the system is suitable for high volume, low cost production with a reliability acceptable for consumer applications of high resolution video imagery.